High smsr unidirectional etched lasers and low back-reflection photonic device

ABSTRACT

Unidirectionality of lasers is enhanced by forming one or more etched gaps in the laser cavity. The gaps may be provided in any segment of a laser, such as any leg of a ring laser, or in one leg of a V-shaped laser. A Brewster angle facet at the distal end of a photonic device coupled to the laser reduces back-reflection into the laser cavity. A distributed Bragg reflector is used at the output of a laser to enhance the side-mode suppression ratio of the laser.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to a method and apparatus forproviding unidirectional emission from an etched laser cavity and forproviding low back-reflection from photonic devices, and in particularto photonic devices coupled to the laser output. More particularly, theinvention relates to V-shaped lasers and triangular ring lasers withetched gaps to control the unidirectionality of emissions at the outputsof the lasers by providing high side-mode suppression and to the use ofan etched facet at or near a Brewster angle to minimize back-reflectionin a photonic device.

2. Description of the Background Art

Advances in current monolithic integration technology have allowedlasers of a variety of geometries to be fabricated, including V-shapedlasers and triangular ring lasers, as described, for example, in AppliedPhysics Letters, 59, pp. 1395-97, 16 Sep. 1991. These developmentsexpand the prospective applications for integrated semiconductor lasersand add the attractiveness of greater manufacturability and reducedcost. This technology has opened the opportunity to explore new andnovel features that can be combined inside and outside the laser cavity.

Copending U.S. patent application Ser. No. 10/226,076, filed Aug. 23,2002, entitled “Wavelength Selectable Device” and assigned to theassignee hereof, the disclosure of which is hereby incorporated hereinby reference, discloses monolithic structures that preventback-reflection from the entrance facet of an element such as anelectroabsorption modulator (EAM) into a laser cavity serving as a lightsource for the element by appropriate selection of the geometry of thedevice. Unidirectional emitting lasers would be desirable in suchconfigurations to maximize the coupling of laser light into an EAM orother such elements.

As is known, a ring cavity laser possess benefits that a Fabry-Perotcavity does not have; for example, a ring cavity will produce lasingaction with higher spectral purity than can be obtained with aFabry-Perot cavity. Monolithic triangular ring lasers as well as othertypes of ring lasers and their integrated couplings have been describedin the prior art, as well as in copending U.S. patent application Ser.No. 09/918,544 filed Aug. 1, 2001, entitled “Curved Waveguide RingLasers,” now U.S. Pat. No. 6,680,961, issued Jan. 20, 2004 and assignedto the assignee hereof, the disclosure of which is hereby incorporatedby reference. In addition, unidirectional behavior in ring lasers alsohas been described in the prior art.

Over the past few years, thanks mainly to the popularity of theInternet, the demand for bandwidth has experienced explosive growth.Carrier companies and their suppliers have addressed this demand byinstalling Wavelength Division Multiplexing (WDM) systems which allowmultiple wavelengths of laser light to be transmitted through a singlestrand of optical fiber. One of the main requirements of lasers used inWDM systems is that the laser sources have spectral purity, with theresult that most of the power of the laser must be concentrated in anarrow wavelength range. A laser source has a large number of possiblelongitudinal modes, and although it has the tendency to operate in theone longitudinal mode that leads to the maximum gain, some other modesare also partially amplified, causing it to generate optical radiationin different wavelengths and reducing its spectral purity.

The use of a gap for improvement of the spectral characteristics of aFabry-Perot laser that operates with standing waves was proposed byLarry A. Coldren and T. L. Koch, “Analysis and Design of Coupled-CavityLasers—Part 1: Threshold Gain Analysis and Design Guidelines,” IEEEJournal of Quantum Electronics, Vol. QE-20, No. 6, pp. 659-670, June1984. As described, a gap was introduced between two cleaved facets toachieve coupling between different cavities, resulting in an improvedmode discrimination depending on the lengths of the cavities and thegap. However, due to the inherent difficulty in building accuratelypositioned cleaved facets and the tolerances involved in placing twocleaved cavities in close proximity to each other to form a gap,large-scale manufacture of these lasers did not materialize. Etchedmirror and groove coupled devices were demonstrated by Larry A. Coldren,et al., in “Etched Mirror and Groove Coupled GaInAsP/InP Laser Devicesfor Integrated Optics,” IEEE Journal of Quantum Electronics, Vol. QE-18,No. 10, pp. 1679-1688, October 1982. However, these etched facets wherenot equivalent in reflectivity to cleaved facets and the etched versionsof these devices were less efficient than their cleaved counterparts.U.S. Pat. No. 4,851,368 taught a process that allowed etched laserfacets to be fabricated that were equivalent in reflectivity to cleavedfacets.

SUMMARY OF THE INVENTION

The present invention is directed to a new and novel technique forobtaining unidirectionality in semiconductor photonic devices such asring lasers and in V-shaped lasers, based on the provision of an etchedgap or gaps in such lasers, and more particularly to the position ofsuch a gap or gaps with respect to the output facet in a ring laser orwith respect to the length of the legs in a V-shaped laser. In addition,this invention is directed to the formation of semiconductor lasers withat least one etched gap to enhance the side-mode suppression ratio(SMSR); i.e., the difference between the power of the main laserwavelength and the side-lobes, in such lasers. Furthermore, theinvention is directed to using a facet formed on photonic devices suchas optical amplifiers, electroabsorption modulators, or the like at ornear the Brewster angle to prevent back-reflection in such a device.

In a preferred form of the invention, unidirectionality in semiconductorlasers is enhanced by forming at least one air gap in the laser cavity,or waveguide, with each gap being defined by spaced apart facets toenhance the side-mode suppression ratio in that cavity. The gaps areprovided, for example, by etching through the cavity of a ridge-typering or V-shaped laser which is integrally fabricated on the surface ofa substrate. The gap or gaps may be etched in any leg of the lasercavity, and may be etched at 90° to the laser axis or, alternatively, atan angle to the axis. In the latter case, the gaps may be etched inspaced-apart pairs, with the waveguide segment between the gaps beingoffset to compensate for refraction at the etched facets. In anotherform of the invention, when the laser output is coupled to a photonicdevice, back-reflection is minimized by the provision of a facet at theBrewster angle at the distal end of the photonic device.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, and additional objects, features, and advantages of thepresent invention will become evident from the following detaileddescription of preferred embodiments thereof, taken with theaccompanying drawings, in which:

FIG. 1 is a schematic illustration of coupled monolithic structuresarranged to prevent back-reflection from interfering with laseroperation;

FIG. 2 is a schematic illustration of a triangular ring laser cavitylaser with two deep etched gap regions in accordance with the presentinvention;

FIG. 3 illustrates a measured spectrum of the triangular ring laser ofFIG. 2;

FIG. 4 is a graphical illustration of the unidirectional behavior of thering laser of FIG. 2;

FIGS. 5( a) to 5(f) illustrate basic configurations for a triangularring cavity with etched gaps in accordance with the invention;

FIGS. 6( a) and 6(b) illustrate double air gaps separated by (a) a flatangle facet air gap and (b) an angled facet air gap; and

FIGS. 7( a) and 7(b) illustrate a V-shaped cavity with DBR reflectors(a) without air gaps and (b) with air gaps.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The disclosure of the application on which this application claims thebenefit under 35 U.S.C. 119(e), U.S. Provisional Application No.60/455,562, filed Mar. 19, 2003, is hereby incorporated herein byreference.

FIG. 1 illustrates a photonic device in the form of a monolithic,unidirectional, triangular ring laser cavity 10 having legs 12, 14, and16, with legs 12 and 16 joining at an exit facet 18. Unidirectionalityis obtained in laser 10 by the provision of an external mirror 20, whichreflects the clockwise light 22 propagating in the laser and emitted atexit facet 18 back into the laser to reinforce the counterclockwiselight 24. A portion of the light 24 is emitted at facet 18 as beam 26,which is coupled into another photonic device 28 such as a semiconductoroptical amplifier (SOA), an electroabsorption modulator (EAM), or thelike. In the illustrated arrangement, the device 28 is a V-shaped EAMwhich includes first and second legs 30 and 32 joined at respectivefirst ends at an output facet 34, with the second end of leg 30 havingan inlet facet 36 positioned to receive emitted beam 26, and the secondend of leg 32 terminating at its extreme, or distal, end at a facet 38,which is at the Brewster angle to minimize reflectivity.

As described in the aforesaid application Ser. No. 10/226,076,undesirable back reflection from an external device such as the photonicdevice 28 may be reduced by locating input facet 36 in such a way as tocouple beam 26 into leg 30 while ensuring that any light 40 reflectedfrom facet 36 will not be coupled back into laser 10. In this manner,the geometry of the device 28 with respect to laser 10 minimizes backcoupling; however, this reduces the amount of light that can be coupledinto the device 28. To maximize coupling of light from the laser to themodulator, direct coupling can be utilized, where the input facet ofdevice 28 directly influences the performance of the laser, for example,by increasing reflectivity.

In accordance with one aspect of the present invention, back-reflectionto a laser from a photonic element such as the EAM 28 is minimized bythe provision, at the extreme distal end of the device 28, of a facet 38at or near the Brewster angle. With this configuration, light 42, whichis coupled into the device 28, propagates in leg 30 and strikes facet 34below the critical angle to cause an output beam 44 to be emitted. Light46, which is the portion of light 42 which is internally reflected atfacet 34, propagates in leg 32, strikes facet 38, and is emitted as beam48 instead of being internally reflected back. In this way, light indevice 28 that is not emitted as output beam 44 will not reflect backinto laser 10, and this will prevent excess chirp from being produced inthe laser. For a complete discussion of modulator facet induced laserchirp, see, for example, “DFB laser with attached external intensitymodulator” by D. Marcuse, IEEE Journal of Quantum Electronics, Volume26, Issue 2, Pages 262-269, February 1990.

Although the laser described in FIG. 1 is a prior art unidirectionalring laser 10, it will be understood that a photonic device 28 withfacet 38 at or near the Brewster angle can be coupled with a variety ofother types of lasers. It will also be understood that the entrancefacet 36 can be at perpendicular incidence to directly couple device 28to the laser.

Another aspect of the invention is illustrated in FIG. 2, wherein atleast one gap is provided in a ring laser cavity 60, and wherein thesize of such a gap and the lengths of the laser elements making up thecavity determine the spectral characteristics of the laser by improvingits side-mode suppression ratio (SMSR) and its unidirectionality. In theillustrated embodiment, the ring laser 60 is triangular, and includeslegs 62, 64 and 66 joined at facets 68, 70 and 72. Facets 68 and 70 arefully internally reflecting, while facet 72 is an outlet facet whichemits counter-rotating light beams 74 and 76. In the illustratedembodiment, two deep gaps 78 and 80 are etched completely through theleg 62, with each having a gap length LG. The length of the segment ofleg 62 between facet 72 and gap 78 is identified as LA; the length ofthe segment of leg 62 between gap 78 and gap 80 is identified as LT; andthe length of the segment of leg 62 between gap 80 and facet 68 isidentified as LB1. The lengths of legs 64 and 66 are identified as LB2and LB3, so the total length LB, or perimeter length, of the opticalcavity is:

LB=LA+LT+LB1+LB2+LB3+2LG

The angle between legs 62 and 66 is selected so that output facet 72 hasan angle smaller than the critical angle, and because of the gaps 78 and80 in leg 62, the counterclockwise (left) output beam 74 is strongerthan the clockwise (right) output beam 76. It will be understood thatthe output beam 74 may be directed to a suitable photonic device such asthe EAM 28 of FIG. 1.

The size of the gaps 78 and 80 and lengths of the various leg elementsforming the ring laser cavity 60 determine the spectral characteristicsof the laser. By adding a single gap such as gap 78, the laser cavity 60is divided into two cavities coupled by an air interface. Using two gaps78 and 80 separated by an intermediate cavity section LT divides thelaser into three cavities: LA, LT, and the combination of LB1, LB2 andLB3, that are coupled successively by the gaps. An example of thespectral efficiency of such a device is illustrated in FIG. 3.

In one form of the invention, the triangular laser cavity 60 is amonolithic ridge waveguide structure fabricated on an indium phosphide(InP) substrate 90, with a total length LB of 350 μm. The cavity issuitably biased, in known manner, to generate light, which willpropagate in both the clockwise direction, indicated by arrow 92, andthe counterclockwise direction, indicated by arrow 94. The legs 62 and66 of the ring laser cavity meet at facet 72 at an angle 96 selected toproduce an angle of incidence of about 12° for light propagating in thecavity.

To produce a high side-mode suppression output from the laser cavity, aswell as unidirectional behavior, at least one air gap is provided in oneleg of the cavity. In the illustrated embodiment, two air gaps areetched through cavity leg 62, each gap being about 3 μm wide, through,for example, photolithography and etching, with the gaps being separatedby a cavity section LT of about 17.5 μm. These etched gaps produceinterference between the cavity sections, which leads to the oscillationof one longitudinal mode in the laser cavity. This produces a side-modesuppression ratio (SMSR) of about 38 dB, as illustrated by curve 100 inFIG. 3, which is the measured spectrum of the light propagating in thecounterclockwise direction to produce the output left beam 74. Theetched gaps result in a substantially unidirectional emission of lightfrom the cavity. As illustrated by the graphs 102 of FIG. 4, the leftbeam 74 light is about five times as intense as the right beam 76 light.

The total length LB of the laser cavity can vary widely and preferablyis between about 10 μm and 10,000 μm. Additionally, the number of gapscan vary, and preferably will be between 1 and 10 gaps positionedanywhere along the length of the ring laser cavity. Each gap preferablyis between about 0.001 μm and about 10 μm in length and will extendcompletely through the cavity.

FIGS. 5( a) to 5(f) illustrate examples of several possibleconfigurations for ring lasers with etched gaps. In these examples, twogaps are used, although it will be understood that fewer or more gapscan be provided, as discussed above. In FIG. 5( a), a triangular ringlaser 120 incorporates two gaps 122 and 124 in a left-hand cavitysegment, or leg, 126, in FIG. 5( b) the gaps 122 and 124 are located ina bottom cavity segment, or leg, 128, of laser 120, and in FIG. 5( c)the gaps 122 and 124 are located in a right-hand cavity segment 130 oflaser 120.

FIGS. 5( c) through 5(f) illustrate laser cavity configurationsutilizing an external photonic element such as a Bragg reflector (DBR)located adjacent the laser outlet facet. In FIG. 5( d), laser 120incorporates gaps 122 and 124 in left-hand segment 126, and includes DBRelement 140 aligned with arm 126 for reflecting the clockwise (right)beam 142 emitted by the outlet facet 144 of ring laser 120. In thiscase, the DBR device is located for reflection into the arm nearest thegaps and results in enhanced unidirectionality. In FIG. 5( e), the DBRdevice 140 is located for reflection into the arm farthest from theetched gaps, and thus is aligned with arm 130 to reflect thecounterclockwise light emitted as beam 146 from facet 144. In this case,the SMSR is enhanced by this structure.

FIG. 5( f) illustrates an embodiment wherein the DBR 140 is located forreflection into either arm 126 or arm 130 when the gaps 122 and 124 arelocated in the central arm 128. This structure is a compromise betweenincreased SMSR and unidirectionality.

Other configurations may also be provided, as by incorporating gaps inboth the left and right arms, with or without DBR devices.

Etched gaps, such as those illustrated at 78 and 80 in FIG. 2 and at 122and 124 in FIGS. 5( a)-5(f) may be formed during the fabrication of thelaser cavity in which they are located, or may be separately fabricatedby etching the substrate before formation of the cavity or by etchingthe cavity after it has been fabricated. Such etching may be carried outusing conventional photolithographic techniques for locating the gapsand for selecting their width.

As illustrated in FIGS. 6( a) and 6(b), the ends of the gaps are definedby facets which the etching process forms in the laser cavity. Thus, forexample, in the embodiment of FIG. 6( a), first and second gaps 150 and152 are fabricated by etching away portions of a laser cavity orwaveguide 154, and forming facets in the waveguide at ends 156 and 158of gap 150 and at ends 160 and 162 of gap 152. These facets may be flat;i.e., at 90° with respect to the axis 164 of the waveguide, in apreferred form of the invention.

In the embodiment of FIG. 6( b), angled facets are formed at the gaps,in which case the adjoining waveguide segments are offset to compensatefor the refraction of light at the interfaces of the laser cavity andthe air gaps. In the illustrated embodiment, two air gaps 170 and 172are provided in a waveguide 174, the air gaps dividing the waveguideinto segments 176, 178 and 180. The ends of air gap 170 are formed byparallel angled facets 182 and 184 in segments 176 and 178,respectively, while the ends of air gap 172 are formed by parallelangled facets 186 and 188 in segments 178 and 180, respectively. Thefacets 182, 184 are angled in a direction opposite to that of facets186, 188, and the centerline of waveguide segment 178 between the gapsis displaced from the centerline 190 of waveguide 174 to compensate forthe refraction of light as it passes throughout the facets into the airgaps and then back into the waveguide. The displacement distance d isrelated to the angle of the facets with respect to the centerline and tothe length l of the air gaps. The angled facets increase the efficiencyof a unidirectional ring laser and improve the intensity of the desiredoutput beam (the left beam in FIG. 2) with respect to the output beambeing suppressed (the right beam in FIG. 2) by suppressing reflectedwaves over the traveling waves propagating in the laser cavity. Althoughtwo gaps are illustrated in FIG. 6( b), it will be understood that adifferent number of gaps can be provided. Further, it will be understoodthat the displacement of the cavity segments with respect to thecenterline of the cavity may be established photolithographically in thelaser fabrication process.

Another embodiment of the invention is illustrated in FIGS. 7( a) and7(b), wherein the ring-type laser discussed with respect to FIGS. 1-6 isreplaced by a V-shaped laser cavity 200. The laser of FIG. 7( a)incorporates first and second legs 202 and 204 fabricated on a substrateat an angle 206 with respect to each other and intersecting at an outletfacet 208, from which light is emitted. The unidirectionality of thelaser is established by the relative lengths of the legs 202 and 204.DBR reflectors 210 and 212 may be provided at the free ends of the legs202 and 204, respectively, to reduce threshold currents and to provideimproved performance.

As illustrated in FIG. 7( b), unidirectionality in a V-shaped lasercavity 220 is enhanced by incorporating at least one air gap in one ofthe intersecting legs 222, 224 making up the cavity. In this embodiment,two spaced air gaps 226 and 228 are located in leg 222, with these gapsincreasing the SMSR in the cavity to provide an improvedunidirectionality at the output facet 230. As illustrated, thisembodiment may also employ DBR reflectors 232 and 234 at the free endsof legs 222 and 224, respectively to improve its performance.

The air gap structure of the present invention serves to significantlyreduce, or prevent, back-reflection when a ring cavity or a V-shapedcavity laser is coupled to another photonic element, such as the EAMdevice described with respect to FIG. 1. Back reflection is furtherreduced by the use of a Brewster angle in the photonic element, as atthe far end of the EAM in FIG. 1.

Although the present invention has been illustrated in terms ofpreferred embodiments, it will be understood that variations andmodifications may be made without departing from the true spirit andscope thereof as set out in the following claims.

What is claimed is:
 1. A semiconductor photonic device, comprising: a substrate; an epitaxial structure deposited on said substrate; a cavity having at least one segment formed in said epitaxial structure, said cavity forming a photonic element selected from the group comprising an electroabsorption modulator and a semiconductor optical amplifier and including an entrance facet and an exit facet; and a laser with a high side mode suppression ratio formed in said epitaxial structure, said laser having an output facet for emitting an output beam that is coupled to said entrance facet of said photonic element; said laser output facet and said photonic element entrance facet being positioned relative to one another so that said output beam is emitted at an angle relative to said laser output facet to reduce back reflection of light from said photonic device to said laser.
 2. The photonic device of claim 1, wherein said cavity is V-shaped and wherein said at least one segment includes a first leg and a second leg.
 3. The photonic device of claim 2, further including an etched facet at or near the Brewster angle at a first, free end of said first leg of said cavity for further reducing back reflections.
 4. The photonic device of claim 3, wherein said first and second legs have second ends that are joined to form said V-shaped cavity, and wherein said exit facet is positioned at a joint formed between said first and second legs.
 5. The photonic device of claim 4, wherein said entrance facet is formed at a first, free end of said second leg of said V-shaped cavity.
 6. The photonic device of claim 1, wherein said laser is selected from the group comprising a ring laser and a V-shaped laser.
 7. The photonic device of claim 6, wherein said laser is a unidirectional laser.
 8. The photonic device of claim 7, wherein said unidirectional laser has an external mirror.
 9. The photonic device of claim 7, wherein said laser includes a plurality of legs and at least one of said legs has at least one air gap formed therein for enhancing the side mode suppression ratio and unidirectionality of said laser. 